What Haskell Teaches Me About Writing Swift

My talk from AltConf 2015

Transcript

1. WHAT HASKELL TEACHES ME ABOUT WRITING SWIFT
   Abizer Nasir | @abizern | abizern.org
   Swift Contractor - Shutl
   

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2. TL;DL
   LEARNING HASKELL WILL MAKE YOU A
   BETTER SWIFT PROGRAMMER.
   

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3. WHY HASKELL?
   

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4. A polymorphically statically typed, lazy,
   purely functional language.
   — The Haskell Wiki
   

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5. TYPE INFERENCE
   

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6. summer :: Int -> Int -> Int
   summer a b = a + b
   λ> :t summer
   summer :: Int -> Int -> Int
   

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7. --summer :: Int -> Int -> Int
   summer a b = a + b
   λ> :t summer
   summer :: Num a => a -> a -> a
   

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8. λ> :i +
   class Num a where
   (+) :: a -> a -> a
   ...
   -- Defined in ‘GHC.Num’
   infixl 6 +
   

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9. FAVOUR
   IMMUTABLE
   STATE
   

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10. ▸ Values don’t change under you.
    ▸ Easier to maintain the code.
    ▸ More likely to be thread safe.
    

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11. ▸ Prefer let to var
    ▸ Prefer structs over classes where possible.
    ▸ Avoid mutating structs.
    

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12. LISTS
    

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14. ▸ Map a function over a list
    ▸ Write the rest of your marvellous program!
    

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15. Don’t iterate over a list
    

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16. HIGHER ORDER FUNCTIONS
    let list = [1, 2, 3, 4]
    let squares = list.map{$0 * $0}
    let evens = list.filter{$0 % 2 == 0}
    let sum = list.reduce(0, combine: +)
    

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17. REDUCE IS OFTEN ENOUGH
    let squares2 = list.reduce([Int]()) {
    var accum = $0
    accum.append($1 * $1)
    return accum
    }
    let evens2 = list.reduce([Int]()){
    if $1 % 2 == 0 {
    var accum = $0
    accum.append($1)
    return accum
    } else {
    return $0
    }
    }
    

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18. MAPPING AND FILTERING IS MORE EXPRESSIVE
    let sumOfSquaresOfEvenNumbers = list
    .filter{ $0 % 2 == 0 }
    .map { $0 * $0 }
    .reduce(0, combine: +)
    

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19. MONADS
    

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20. A monad is a monoid in the category of
    endofunctors, what’s the problem? --
    Philip Wadler1
    1 Not really – see http://james-iry.blogspot.com/2009/05/brief-incomplete-and-mostly-wrong.html
    

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21. A MONAD IS
    (SORT OF) LIKE A
    BURRITO
    

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22. A VALUE IN A
    CONTEXT
    

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23. ▸ Optional is a context that contains a value or the lack of a value
    

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24. ▸ A list is a context that contains values in an order
    

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25. ▸ Result is the context that has a value or an error in getting that
    value
    

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26. THE TYPE SYSTEM
    IS YOUR FRIEND
    

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27. There is a whole class of bugs we no longer have to worry about.
    

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28. "If it compiles, you're most of the way to your solution"
    

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29. typealias is useful for making code clearer.
    func decode(json: [String : AnyObject]) -> MyType { ... }
    or
    typealias JSONDictionary = [String : AnyObject]
    func decode(json: JSONDictionary) -> myType { ... }
    

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30. It can make higher order function declarations easier to read.
    // Taken from AlamoFire
    typealias JSONResponseHandler = (NSURLRequest, NSHTTPURLResponse?, AnyObject?, NSError?) -> ()
    // Define a common completion block with specific handler
    func responseHandlerWithCompletion(completion: Result) -> JSONResponseHandler { ... }
    

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31. ▸ You can reason about your problem in terms of the types and
    transformations of those types, before writing any code.
    

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32. TYPE DRIVEN DEVELOPMENT
    

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33. func shuffle(deck: Deck) -> Deck { ... }
    func deal(deck: Deck, toPlayers: [Player]) -> [Hand] { ... }
    func winner(hands: [Hand]) -> Player { ... }
    

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34. KEEP CODE WITH
    SIDE EFFECTS IN
    ONE PLACE
    

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35. module Main where
    import Control.Monad
    import Numeric
    factoryTimes :: Double -> Double -> [Double]
    factoryTimes c f = 0.0 : [ c / (2.0 + k * f) | k productionTimes :: Double -> Double -> [Double]
    productionTimes x f = [ x / (2.0 + k * f) | k times :: Double -> Double -> Double -> [Double]
    times c f x = zipWith (+) production factory
    where production = productionTimes x f
    factory = scanl1 (+) $ factoryTimes c f
    firstMinimum :: [Double] -> Double
    firstMinimum (x:y:ys) = if x < y
    then x
    else firstMinimum (y:ys)
    solve :: Double -> Double -> Double -> Double
    solve c f x = firstMinimum $ times c f x
    main :: IO ()
    main = do
    t forM_ [1..read t :: Int] $ \i -> do
    [c, f, x] let result = solve c f x
    putStrLn $ concat ["Case #", show i, ": ", Numeric.showFFloat (Just 7) result ""]
    

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36. FUNCTIONS ALL
    THE WAY DOWN
    

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37. ▸ Don't have state, have functions that return values; although this is
    easier said than done.
    ▸ Higher order functions can be used to reduce duplication in code.
    

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38. ▸ Aim to write small, pure functions with no side effects.
    ▸ Avoid mixing pure and impure code.
    ▸ Aim to write small, obviously correct functions.
    

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39. Composition
    infix operator >>> { associativity right precedence 170 }
    func >>> (f: B -> C, g: A -> B) -> A -> C {
    return { x in f(g(x)) }
    }
    

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40. Map on Optionals
    infix operator { associativity left }
    func (f: A -> B, a: A?) -> B? {
    if let x = a {
    return f(x)
    } else {
    return nil
    }
    }
    

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41. Applicative on Optionals
    infix operator { associativity left }
    func (lhs: (A -> B)?, rhs: A?) -> B? {
    if let f = lhs {
    if let x = rhs {
    return f(x)
    }
    }
    return nil
    }
    

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42. Bind on Optionals
    infix operator >>== { associativity left precedence 150 }
    func >>== (a:A?, f: A -> B?) -> B? {
    if let x = a {
    return f(x)
    } else {
    return nil
    }
    }
    

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43. DO WE STILL
    NEED OOP?
    

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44. OOP concerns data objects and
    operations on those data objects
    — My naive definition
    

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45. "Why are methods of protocols defined outside the Type?"
    struct SomeType: Equatable { ... }
    func ==(lhs: ...
    

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46. If we are supposed to favour immutability, shouldn't we be moving away
    from classes?
    

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47. Have you ever created a class with nothing but class functions just to
    keep related code together?
    

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48. Have you ever carefully exposed only the methods that were safe to call
    on your objects, keeping other methods private?
    

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49. If you have pure functions (no side effects, same inputs, same outputs)
    is it worth worrying about their access modifiers?
    Can you make them bare functions instead?
    

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50. If the functions can be grouped together into a logical set of
    functionality, could a protocol be defined to encompass that
    functionality instead?
    

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51. Programming to an interface was a recommendation even in the times
    of Objective-C. The stronger type system in Swift makes it even more
    attractive.
    

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52. REFERENCES
    

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53. Learn You a Haskell For A Great Good
    http://learnyouahaskell.com
    

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54. Real World Haskell
    http://book.realworldhaskell.org
    

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55. Functional Programming in Swift
    http://www.objc.io/books/fpinswift/
    

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56. Maybe Haskell
    https://gumroad.com/l/maybe-haskell/
    

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57. LEARNING HASKELL WILL MAKE YOU A
    BETTER SWIFT PROGRAMMER
    

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58. LEARNING A PROGRAMMING LANGUAGE WILL
    MAKE YOU A BETTER PROGRAMMER
    

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59. WITH COCOA WE HAVE THE CHOICE OF TWO
    DIFFERENT APPROACHES TO THE SAME
    PROBLEM SPACE.
    

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60. UNFAMILIARITY
    CAN BE MISTAKEN FOR
    COMPLEXITY
    

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61. I want to write code like this:
    static func decode(object: JSON) -> DateWithTimeZone? {
    return jsonDictionary(object) >>== { dict in
    DateWithTimeZone.create
    dict[dateWithTimeFormatKey(.Date)] >>== jsonDate
    dict[dateWithTimeFormatKey(.TimeZone)] >>== jsonTimeZone
    }
    }
    It looks complex because it's unfamiliar, but it's clearer than having a
    pyramid of doom.
    

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62. This stuff is hard. But compared with what we already do and what we
    aspire to do; it's perfectly within our grasp.
    

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63. THANK YOU
    &
    HAVE A GREAT WEEK!
    ABIZER NASIR | @ABIZERN | ABIZERN.ORG
    

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